Q: There are several cats on a sloping roof. Which one falls off first?

Q: There are several cats on a sloping roof.  Which one falls off first?
A: The one with the lowest mew!

This is a pun as well as a physics joke. The joke lies in the fact that mew (μ) can mean the sound that a cat makes, and it can also be (among other things) the symbol used to represent the coefficient of friction.

The cat with the lowest coefficient of friction would slide off the roof first if all other variables were the same, however the positioning of the cats, and local conditions surrounding the cats could reasonably make the cats not fall off the roof exactly according to their μ.